Project Summary The CDC estimated that one-quarter of Americans living with HIV were over the age of 55 in 2012. By next year, these individuals will be over age 60, entering into the age demographic where Alzheimer's disease (AD) becomes a distinct differential for clinicians. Because up to one-half of people living with HIV experience cognitive impairment from HIV or related factors, the likelihood for masking and thus delaying the diagnosis of early AD is substantial. Differentiating HIV-associated Neurocognitive Disorder (HAND) from the Mild Cognitive Impairment stage of AD (MCI-AD) is one of the most pressing issues in geriatric neuroHIV. Current HAND nosology does not provide guidance on this issue. Published work suggests the likelihood for distinct phenotypes that would facilitate diagnostic sorting with commonly available inputs from neuropsychological testing and structural imaging, but these may not be sufficient. Many studies highlight the importance of myeloid cells (monocytes, macrophages and microglia) in the pathogenesis of both HAND and AD. In this application we propose to harness the power of epigenetics utilizing cell type specific signatures and cutting- edge single cell technologies to define distinguishing and overlapping immune biomarkers of HAND from the MCI-AD in individuals over 60 years of age. This application will test the hypothesis that a rare population of circulating monocytes in blood defined by an indelible epigenetic profile related to the CNS are present in HAND compared to age-matched cognitively normal HIV+ individuals over age 60 on suppressive anti- retroviral therapy. We further posit that comprehensively defining this HAND-related monocyte cell type will aid in distinguishing HAND from individuals with MCI-AD while also defining fundamental neuropathogenic mechanisms of each disease. The specific aims are Aim 1: To examine monocyte epigenetic phenotypes in individuals over 60 years with early MCI-AD compared to HAND and, separately, cognitively normal age-matched HIV+ individuals on suppressive ART. Aim 2 To utilize single cell resolution technology to interrogate unique monocyte subpopulation phenotypes that can be used to compare and contrast with MCI-AD and HAND. Aim 3: To validate MCI-AD and HAND related signatures in primary monocytes and define functional consequences of site-specific methylation events. Two major outcomes are expected from the knowledge gained by this proposal: (1) to reveal biological pathways to understand the pathogenesis of HAND and MCI-AD. and 2) address the challenges of distinguishing HAND from Mild Cognitive Impairment due to AD in geriatric neuroHIV.